The present invention generally relates to head-up display (HUD) systems and, more particularly, to a method and apparatus for attenuating solar radiation entering a HUD.
Head-up displays (HUD) are known and have been used in military and commercial aircraft for several years. HUDs have also found application in automobiles. In the automobile application, a HUD unit typically projects an image onto the windshield in front of the driver along the driver""s forward line of sight. The HUD""s projection optics are conventionally mounted in the instrument panel below the windshield. An advantage of HUDs is that a driver can view the display without taking his or her eyes off of the road. In some embodiments, the HUD projection results in a virtual image that appears to be positioned at a distance in front of the vehicle. This is useful, for example, in a night vision HUD that projects a simulated view of the road and obstacles ahead of the vehicle. This provides an early warning to the driver. Adapting HUD units for use in automobiles requires making the units rugged, reliable, cost-effective, and functional in a wide range of environmental conditions.
A problem with HUDs in automobiles is high thermal loads due to solar radiation. During bright sunny days, solar radiation enters the HUD system causing thermal stress to the internal components. Some of the components which are sensitive to this thermal stress include the display, the electronics, the lens, and the HUD case. The problem is intensified due to the size, reflectivity, and magnifying characteristics of the HUD projection optics (e.g., the aspheric mirror). The HUD optics can magnify the solar radiation by a factor of ten or more. The problem experienced with high solar thermal radiation in an automobile HUD is illustrated in FIG. 1.
In a typical HUD configuration, solar radiation 12 (i.e., sunlight) enters through the windshield 13 of vehicle 10. Radiation 12 enters the HUD 11 via lens 14 where it is reflected and magnified by mirror 15. Mirror 15 is one element of the projection optics of the HUD. In the worst case scenario, radiation 12 is focused on a thermally sensitive component such as display unit 16. The HUD projection optics (e.g., mirror 15) can magnify radiation resulting in temperatures exceeding 140 degrees Celsius. This thermal stress can damage HUD components.
The prior art has attempted to solve this problem in numerous ways. One technique uses a filtering lens 14 to reduce the thermal effects of solar radiation 12. The filtering lens 14 reduces the thermal radiation entering the HUD and thereby reduces the thermal stress. However, this conventional technique is generally not sufficient to solve the problem alone and is therefore typically used in combination with some of the following techniques.
Another conventional technique for reducing the adverse affects of high solar thermal radiation is to use heat shields. Heat shields are typically made of metal and shield sensitive components from the radiation 12. The shields generally work reasonably well, but are expensive and cannot be used to shield the display unit 16 at all times.
A third technique to handling this radiation problem is very expensive and involves the use of a shutter mechanism to shield the display unit 16 from the radiation 12. The shutter mechanism comprises moving parts that are moved into a position to shield display unit 16 as needed. The shutter mechanism also requires actuators and electronics to control the shielding operation. This solution is obviously costly and may have lower reliability.
A fourth technique to avoiding adverse affects of thermal radiation uses heat tolerant components and materials. However, conventional heat tolerant items are generally more expensive. For example, the HUD case or lens 14 can be made of less expensive plastics if they are not required to tolerate high thermal stress.
Accordingly, it is desirable to provide for a HUD that has improved operation in high intensity solar radiation conditions. It is also desirable to provide a HUD system design that does not require costly shields, shutters, lenses, and thermal tolerant components, particularly for use in an automobile.
The present invention provides for a head-up display (HUD) that couples reduced reflectivity projection optics and a high brightness light source to produce a high quality image and reduce the thermal effects of solar radiation on HUD components.
The HUD generally includes a display, a light source, a mirror, and a combiner. The light source illuminates the rear of the display, which projects an image to the combiner via the mirror. The invention reduces the thermal load on HUD components by reducing the reflectance of the mirror. The light source of the invention is preferably brighter than a conventional backlight and thus compensates for a portion of the reduced reflectance of the mirror.
The invention calls for a mirror that has reduced reflectance. The preferred embodiment uses a mirror having reflectance of approximately 30%. In contrast, this is about ⅓ the reflectivity of the mirrors in the prior art. Historically, the prior art taught the opposite of the invention by attempting to maximise mirror reflectivity and specified a high reflectivity of typically 88% to 90%. This is largely because of the light loss in other elements of the conventional HUD system. For example, one factor contributing to light loss is the windshield (i.e., combiner) in the automobile. Government regulations generally prevent windshields from being coated or tinted to reflect the projected image of a HUD. Consequently, a typical untinted windshield reflects approximately only 15% of the projected image brightness from a HUD. Another factor contributing to light loss is the display. Liquid crystal displays (LCDs) typically used in HUDs transmit only approximately 8% of the light from a backlight light source. Given these light loss factors and others, it was generally presumed in the prior art that the mirror reflectance should be as high as possible to prevent any further loss of light. The invention runs counter to these teachings and reduces the reflectance of the mirror.
The preferred embodiment of the present invention uses a single aspheric mirror in the projection optics, however, those skilled in the art understand that multiple mirrors, whether aspheric or not, may also be used. The invention is equally applicable to HUDs having projection optics with multiple mirrors. In this specification, the term reflector system is defined to include a single mirror or multiple mirror systems for reflecting the image generated by the HUD.
A new high brightness light source backlights the display. The light source is approximately three times as bright as similar light sources in the prior art. The high brightness light source compensates for a portion of the reduced reflectance of the reflector system such that the resulting projected image retains approximately the same illumination level. The higher cost of the new light source is more than compensated for by the cost savings in the other components of the HUD.
The present invention may use a conventional display. The display forms symbology on its screen responsive to the video signal from the video processor. Because the light source is brighter, the resulting projection image from the display is also proportionally brighter. The image is directed to the reflector system that reflects the image to the windshield.
According to one aspect of the present invention, the windshield functions as the combiner in a typical automotive HUD application. The HUD generated image is projected onto the windshield where it is in the forward line of sight of the driver. It is common for automotive applications to use the windshield as the combiner. However, it is understood that the invention is equally applicable to HUD systems that use a conventional combiner element. For purposes of this specification a combiner refers to any element that serves as a display screen for the HUD image and includes a conventional combiner, windshield, or similar element.
Referring to FIG. 1, the operation of the invention can be understood. Solar radiation 12 passes through windshield 13 and lens 14 and enters HUD 11. Because of the reduced reflectance mirror of the invention, lens 14 may be less expensive since it need not filter the solar radiation 12 nor withstand high thermal stress. The radiation 12 is reflected and magnified by mirror 15. Because of the invention, however, mirror 15 reflects only approximately 30% of the solar radiation 12. The intensity of the reflected solar radiation is reduced sufficiently so that costly heat shields and other elements may be reduced or eliminated.
The design of the present invention advantageously solves the problems of the prior art to provide an improved solar radiation tolerant HUD. Reducing the reflectance of the reflector system results in reduced thermal loading of the HUD components. The reduced thermal loading allows elimination of components such as mechanical shutters, thermal shields, and filtering lenses. The invention may also result in reduced assembly costs, reduced parts costs, and increased reliability.
Yet another discovered advantage of the invention is a reduction in green or colored reflections onto the windshield 13. Prior to the invention, reflection was caused by the typically green tint applied to lens 14. Light reflected by lens 14 caused a green reflection on the windshield. These reflections are generally considered annoying and unsightly. Because the invention eliminates the requirement for a tinted lens 14, this problem may also be eliminated.
It is envisioned that the invention may be adapted to various embodiments of HUD systems. This includes HUDs with multiple mirror projection optics, various light sources, various combiners, and various optical path configurations.
These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.